5-(and-6)-Carboxy-2',7'-dichlorofluorescein diacetate, succinimidyl ester is a useful fluorescent tracer that can passively diffuse into cells and covalently label intracellular proteins, resulting in long-term cell labeling. The reagent itself is colorless and nonfluorescent but becomes brightly green fluorescent once it is hydrolyzed by intracellular esterases. The fluorescence of the dye is relatively insensitive to pH change and the labeling has been shown to be stable to formaldehyde or gluteraldehyde fixation. &lamba;Ex/&lamba;Em (of hydrolysis product at pH 7) = 504/529 nm White solid soluble in DMSO or DMF Store at -20°C and protect from light C29H17Cl2NO11 MW: 626